Wind turbines always incorporate a rotor, that is to say some form of vane wheel or blade wheel. This rotor may be exposed or may be connected to a ducting. By the use of such a ducting of appropriate design it is possible to produce an accelerated wind flow past the rotor. This may be achieved in two ways, either by means of an accelerator which is positioned ahead of the rotor and which has the effect of increasing the velocity of the air flow as it arrives and before it reaches the rotor, or by means of a diffuser which is positioned behind the rotor and which contributes to leading away the air flow behind the rotor so that said air flow does not constitute an impediment to the arriving air flow. The principle may also be explained in terms of the fact that the accelerator attempts to increase the dynamic pressure ahead of the rotor, leading to an increase in the velocity of the arriving air. The diffuser for its part distributes the air leaving the turbine over a greater volume by causing the air to be diffused, whereby the dynamic pressure will fall in spite of the fact that the static pressure may rise as a result of the reduction in the velocity of the air which takes place. It is usual for a turbine ducting to be provided with both an accelerator and a diffuser arrangement.
The use of such a velocity-increasing arrangement permits the wind to be utilized more effectively, especially in the case of low wind speeds. A conventional wind turbine must, in actual fact, be so executed as to be capable of withstanding the maximum wind force occurring at the place of installation or, in the event of means being provided for taking the turbine out of service at a certain wind force, of withstanding the maximum wind speed at which it is proposed to operate. This means that the rotor will possess a certain starting resistance, with the result that the turbine will not generate any power in low winds. The use of a ducting arrangement enables the limit value for the lowest wind speed to be reduced partly because the velocity of the wind is accelerated and partly because the rotor is able to be made smaller than in the case of an exposed rotor because of the higher wind velocity at which it operates, with the result that it is not exposed to the risk of damage to the same extent.
The smaller size of the rotor may also result in a lower manufacturing cost since certain parts of the rotor are replaced by a static component which can be made not only more robust, but often at a lower cost than the component which is to rotate. The power produced by a wind turbine bears a certain relationship to its frontal area, which is constituted only by the rotor in the case of an exposed turbine, whereas in the case of a ducted turbine it is constituted by the rotor together with the non-rotating ducting.